Power tools are known which comprise a body which houses a motor and an attachment for coupling with the body in order to form a certain task such as drilling or sawing of a workpiece. The attachment is usually task-specific and so will generally need to be adapted for the task.
An example of such a power tool is shown in EP-A-899,063, whereby a composite power tool is formed from a body and any one of a plurality of attachments. The body houses an electric motor for supplying a driving force to the attachment mounted on the body, whereby the body of the tool does not house a gear mechanism and only a variable switch may be used to control the output speed of the motor. An attachment, such as a drill head, for example, may include its own gear mechanism due to the fact that the speed control of the motor may be across the whole range of speeds from still to maximum output speed. In this manner, the mechanism may only control across a small window of speeds. Alternatively, the accuracy of control of the motor speed by a user may not be very good due to vibration of the tool during use.
For the above reasons, therefore, it has been known to employ a gear mechanism in certain attachments in order to have a step reduction in speed as between the output of the motor and the output of the attachment itself.
The above still presents problems, however. Although certain attachments may include gear mechanisms to step down the input rotational speed, the output of the motor is ungeared and directly applied to the input of the attachment which may, or may not be geared.
When considering the desired rotational (or reciprocating) speed of various attachments such as sanders, jigsaws or drills, for example, a wide range can be seen. For example, a drill may rotate at up to 2–3,000 rpm, whilst a jigsaw may have a reciprocal movement of 1–2,000 strikes per minute. On the other hand a sander may need an orbital rotation of 20,000 rpm.
Clearly, to cater for such a vast range of output speeds would require a large gear mechanism (probably a large, multi-stage gearbox) in each attachment, if the attachment is driven directly from the motor output.
However, if the motor output can itself be geared, then each attachment may only need a relatively small, simple gear mechanism of its own in order to become well tuned to its specific task.